The present application is directed to a fusion cage for use as a spinal implant between a pair of vertebrae in order to provide support to and subsequently promote the fusing together of the vertebrae and, in particular, a threaded fusion cage having a thread that is feathered near an anterior end of the fusion cage, such that a minor diameter of the thread approaches a major diameter of the thread near the anterior end.
Fusion cages are commonly implanted between a pair of vertebrae of the spine in order to provide support to the vertebrae and to eventually promote fusion between the vertebrae. Fusion cages are generally of two types which are rectangular in cross-section or circular in cross-section. The present application is directed to the latter type which are overall generally cylindrical in shape and threaded so as to provide a better grip for the cage upon implantation. The threaded cages are normally inserted between vertebrae where the disc between the vertebrae has been removed and a partial bore has been provided by drilling or the like. Normally, the fusion cages will have windows extending between the top and bottom thereof to allow bone to grow through and fuse together between the vertebrae. Also the interior of the fusion cage is often packed with bone or other matrix that encourages the growth of bone into the cage and between the two vertebrae and, therefore, a subsequent fusing therebetween.
The facing surfaces of the vertebrae between which the cages are inserted are mostly of a relatively soft spongy bone. Because of this, the cages are subject to subsiding into the bone either very soon after surgery or at a later date. Such subsidence is undesirable.
While the majority of the bone adjacent the facing surfaces of the vertebrae is spongy and somewhat soft, a comparatively narrow band of bone at the anterior of each vertebrae is comparatively harder bone. It is, therefore, desirable to take advantage of this harder bony region by improving the contact of the fusion cages, as much as possible, in this region and reducing structure into which the bone can subside.
A fusion cage has a cylindrical or partially cylindrical shape with a pair of semi-circular surfaces on opposite sides thereof that are sized and shaped to engage facing surfaces of a pair of vertebrae that are to be supported and fused by the fusion cage. The fusion cage surfaces include a thread thereon that may be continuous about the fusion cage, but is normally only on the surfaces that engage the vertebrae during actual use and, in such cases, a complementary thread may be provided by an insertion tool to allow the device to be threadedly advanced into a bore located between the two adjacent vertebrae. For example, see U.S. Pat. No. 5,865,847 of Kohrs et al. which is incorporated herein by reference.
The thread has a major diameter and a minor diameter. Over most of the length of the cage and especially near the posterior end of the cage, the minor diameter is substantially less than the major diameter thereby forming a series of peaks and valleys constituting the threadform. The major diameter of the thread stays generally constant along the entire length of the thread. The thread depth remains generally constant near the posterior end and middle of the cage, but is reduced near the anterior end of the cage.
In particular, located near the anterior end of the cage and sized and positioned to engage an anterior surface of comparatively harder bone near the anterior of the vertebrae is a modified thread. In the modified thread the minor diameter approaches and preferably equals the major diameter at the front or anterior end of the cage to effectively reduce the thread depth in this region. Preferably, the minor diameter over a short distance approaches the major diameter evenly or progressively, as the threadform winds discontinuously, but in a helical form or pattern about the cage. Also preferably the increased minor diameter occurs over approximately at least the front xc2xc inch of the thread, while the major diameter remains constant. Further, the minor diameter is preferably approximately equivalent to or the same height as the major diameter between the final two passes of the threadform near the anterior end of the cage so as to present a generally smooth cylindrical or semi-cylindrical surface in this region.
Therefore, the objects of the present invention are: to provide a fusion cage for implantation between a pair of vertebrae, so as to provide support and fusion between the vertebrae, and that resist subsidence into the vertebrae by providing as much support as possible near an anterior end of the fusion cage whereat the fusion cage engages a harder bony region of the vertebrae; to provide such a cage that is generally cylindrical in shape and threaded; to provide such a cage having a thread with a major diameter and a minor diameter that is helical in form, but is not continuous, that is wound about the cage; to provide such a cage wherein the thread is feathered near the anterior end of the cage by increasing the minor diameter of the thread, while maintaining the major diameter constant, until the minor diameter approaches or equals the major diameter of the thread, so as to reduce thread depth at the anterior end of the cage; to provide such a cage having a generally smooth cylindrical surface at the anterior end thereof that is at least the equivalent of several thread turns in width; and to provide such a cage wherein the minor diameter of the thread is allowed to increase evenly and progressively over approximately xc2xc of an inch length of the cage until it approaches or is equal to the major diameter; to provide such a cage which is comparatively easy to construct, easy to use and especially well adapted for the intended purpose thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.